I think you want to add $HOME/openmpi_`uname -m`/lib to your
LD_LIBRARY_PATH. This should allow executables created by mpicc (or
any derivation thereof, such as extracting flags via showme) to find
the Right shared libraries.

Let us know if that works for you.

FWIW, we do recommend using the wrapper compilers over extracting the
flags via --showme whenever possible (it's just simpler and should do
what you need).